Apparatus of the type in question, which are well known in the art, are made either in the form of a bare filament in a vacuum or not, or coated with refractory material, or in the form of a surface deposit of an electro-resistant material on an electric insulation, so that it is capable of attaining a temperature of between 1000.degree. and 2500.degree. C. by circulation of an electric current. This generator emits infra-red rays of short wave-length, i.e. of which the majority of the emission lies between 0.75 and 3 microns.
Such electric generators are used either for electrical heating or for industrial heating, for example for treating a coating on a support such as a paint with a view to the baking thereof.
It has been observed that the materials to be treated are much more transparent to short infra-red rays which therefore pass through the coating in question, so that it is not treated in all its thickness.
It is also known that, in order suitably to treat the whole thickness of a coating, it is necessary to emit a broad spectrum of infra-red rays, i.e. of which the wave lengths vary from about 0.75 to 10 microns.